1. Field of the Invention
The present invention relates to the control of synchronous electric machines, notably for motor vehicles.
2. Description of the Prior Art
A synchronous machine is comprised of a rotating part which is the rotor and of a stationary part which is the stator. The rotor can be made up of permanent magnets or of a DC-powered coil and a magnetic circuit which is referred to as the electromagnet. The stator comprises three phases with each phase having at least one coil (also referred to as winding) connected thereto with the coils being supplied with electrical power and voltage. An external force is used to rotate the rotor which is a magnetic field induced by an alternating electric current in the coils (windings) of the stator which causes the rotor to rotate. The speed of this rotating field is referred to as the “synchronous speed”.
To control synchronous electric machines, it is important to know in real time the angular position and the speed of the rotor. Indeed, the position information is used conventionally to provide vector control of the torque of the machine. Vector control is referred to as controlling the machine to produce the torque required by the application which requires the electric currents circulating therein to be maintained in phase and in synchronism with the position of the rotor. The control of the electric machine therefore applies voltages at the terminals of the motor, which are provided by a torque control algorithm.
Position detectors, of either the Hall effect or inductive type for example, are commonly used to determine the position of the rotor. However, low-cost position detectors are not accurate enough, notably for high rotating speeds, and they therefore do not allow precise control of the torque of the electric machine. Furthermore, these detectors can be subject to failure or involve measurement noise, thus generating measurement uncertainties. The use of a position detector is for example described in British patent application 1,214,331 A. Another option uses precise position detectors such as high-resolution incremental encoders or absolute detectors called resolvers which are detectors based on the detection of a rotating magnetic element, or on an optical interferometry principle. However, these position detectors have the major drawback of being expensive.
Another solution of the prior art is the reconstruction of the position of the rotor by estimating, from the electric measurements, a physical quantity that varies with the position of the rotor. The position estimations can be classified into two major categories which are:
Those based on the injection of particular signals in the electric machine control, which require applying particular voltages at the terminals of the electric machine so that the position can be determined from electric measurements on the motor. For example, French patent application 2,623,033 A1 describes the injection of short pulses in two non-energized phases. The injection of particular signals involves constraints regarding the electric machine control, which do not enable optimum control of the motor during operation thereof.
Those requiring no particular signal at the motor input, which are based only on a mathematical description of the behavior thereof by a real-time estimator, are also referred to as an observer. But this technique has the drawback of not delivering a precise estimation in a case of near-zero or low motor speed. For example, French patent application 2,781,318 B1 discloses a method of estimating the rotation angle of the rotor through calculation from signals provided by voltage sensors. U.S. published patent application 2010/237,817 A describes the use of an observer utilizing an equivalent electromotive force model.
In order to overcome the drawbacks of the prior solutions, there are known “hybrid” solutions using a position detector for low rotating speeds, as well as an estimation method for high speeds. For example, the assignee's French patent application 11/03,994 describes such a hybrid solution with an algorithm allowing determination of the position and the speed of the motor for high rotating speeds. However, these hybrid solutions always require using a detector for low electric machine speeds. Furthermore, the prior art solutions do not have sufficient precision for control of electric machines. Indeed some prior art solutions make approximations prior to developing observers which are referred to as Kalman filter observers. Moreover, some of these Kalman filter observers require many complex calculations.